To make the ice cream:
In a non-reactive saucepot, whisk together the cream, cocoa, sugar, and salt. Mix until smooth and heat over medium heat until just simmering.
Add the chopped chocolate and whisk gently until completely smooth. Remove from heat. Add the milk and zest, if using, and whisk to combine.
Pour the chocolate mixture into a bowl (strain if you added the zest), cover with plastic and refrigerate until completely chilled, about 3 hours or overnight.When the mixture is chilled it will be much thicker.
Freeze according to your ice cream maker's instructions.